Gamma-ray detectors for downhole applications

Abstract

Methods and related systems are described for gamma-ray detection. A gamma-ray detector is made depending on its properties and how those properties are affected by the data analysis. Desirable properties for a downhole detector include; high temperature operation, reliable/robust packaging, good resolution, high countrate capability, high density, high Z, low radioactive background, low neutron cross-section, high light output, single decay time, efficiency, linearity, size availability, etc. Since no single detector has the optimum of all these properties, a downhole tool design preferably picks the best combination of these in existing detectors, which will optimize the performance of the measurement in the required environment and live with the remaining non-optimum properties. A preferable detector choice is one where the required measurement precision (logging speed) is obtained for all of the required inelastic elements and/or minimization of unwanted background signals that complicate the data analysis.

What is claimed is: 
     
       1. A system for detecting gamma-rays downhole comprising:
 a tool housing adapted and dimensioned to be deployed in a borehole within a subterranean formation; and 
 a scintillator material mounted within the tool housing, and emitting light when gamma-rays are absorbed, the scintillator material comprising a lanthanum bromide material. 
 
     
     
       2. A system according to  claim 1  further comprising a photodetector mounted within the tool housing and adapted so as to detect light emitted by the scintillator material. 
     
     
       3. A system according to  claim 1  wherein the system is adapted to detect gamma-rays primarily produced by inelastic scattering. 
     
     
       4. A system according to  claim 3  wherein the system is adapted to reduce detection of gamma-rays produced by neutron capture events. 
     
     
       5. A system according to  claim 1  wherein the system is adapted to detect gamma-rays primarily produced by neutron capture events. 
     
     
       6. A system according to  claim 1  further comprising a nuclear source mounted within the tool housing and adapted so as to emit nuclear radiation into the subterranean formation. 
     
     
       7. A system according to  claim 6  wherein the nuclear source includes a neutron source adapted to emit neutrons into the subterranean formation so as to produce gamma-rays. 
     
     
       8. A system according to  claim 7  wherein the neutron source is an electronic neutron generator. 
     
     
       9. A system according to  claim 8  wherein the system is adapted to detect gamma-rays primarily produced by inelastic scattering and reduce the sensitivity to epithermal neutrons to enhance the measurement of elements. 
     
     
       10. A system according to  claim 7  wherein the neutron source is a pulsed neutron source. 
     
     
       11. A system according to  claim 1  wherein the spacing between the nuclear source and the scintillator material is based at least in part on the selection of the scintillator material. 
     
     
       12. A system according to  claim 1  wherein the tool housing is adapted to be deployed in the borehole via a wireline cable. 
     
     
       13. A system according to  claim 1 , further comprising:
 a data processing system adapted to determine one or more properties of the subterranean formation based at least in part on the signal, wherein the data processing system is adapted to use least-squares processing. 
 
     
     
       14. A method for detecting gamma-rays downhole comprising:
 deploying a tool in a borehole within a subterranean formation; and 
 making gamma-ray measurements using a scintillator material mounted in the tool, the scintillator material emitting light when gamma-rays are absorbed, wherein the scintillator material comprises a lanthanum bromide material. 
 
     
     
       15. A method according to  claim 14  wherein the gamma-ray measurements are made further using a photodetector mounted within the tool and adapted so as to detect light emitted by the scintillator material. 
     
     
       16. A method according to  claim 14  wherein the method is adapted to detect gamma-rays primarily produced by inelastic scattering. 
     
     
       17. A method according to  claim 14  wherein the method is adapted to detect gamma-rays primarily produced by neutron capture events. 
     
     
       18. A method according to  claim 14 , further comprising:
 determining one or more properties of the subterranean formation based at least in part on the signal, wherein the determining comprises least-squares processing. 
 
     
     
       19. A system for detecting gamma-rays downhole comprising:
 a tool housing adapted and dimensioned to be deployed in a borehole within a subterranean formation; 
 a scintillator material mounted within the tool housing, and emitting light when gamma-rays are absorbed, the scintillator material including a lanthanum bromide material; and 
 a photodetector mounted within the tool housing and adapted so as to detect light emitted by the scintillator material, the scintillator and the photodetector forming portions of a gamma ray detector, the system being adapted to detect gamma-rays that are primarily produced by neutron capture events. 
 
     
     
       20. A system according to  claim 19  further comprising a neutron source mounted within the tool housing and adapted to emit neutrons into the subterranean formation so as to produce gamma-rays. 
     
     
       21. A system according to  claim 20  wherein the neutron source is an electronic neutron generator. 
     
     
       22. A system according to  claim 20  wherein the neutron source is a pulsed neutron source. 
     
     
       23. A system according to  claim 19  wherein the tool housing is adapted to be deployed in the borehole via a wireline cable. 
     
     
       24. A method for detecting gamma-rays downhole comprising:
 deploying a tool in a borehole within a subterranean formation; and 
 detecting gamma-rays that are primarily produced by neutron capture events using a scintillator material and a photodetector mounted within the tool, the scintillator material including an lanthanum bromide material and emitting light when gamma-rays are absorbed, and the photodetector detecting light emitted by the scintillator material. 
 
     
     
       25. A method according to  claim 24  further comprising emitting neutrons into the subterranean formation using an electronic neutron generator mounted within the tool, so as to produce gamma-rays. 
     
     
       26. A system for detecting gamma-rays downhole comprising:
 a tool housing adapted and dimensioned to be deployed in a borehole within a subterranean formation; 
 a scintillator material mounted within the tool housing, and emitting light when gamma-rays are absorbed, the scintillator material including lanthanum and bromine; and 
 a photodetector mounted within the tool housing and adapted so as to detect light emitted by the scintillator material, the scintillator and the photodetector forming portions of a gamma ray detector, the system being adapted to detect gamma-rays that are primarily produced by inelastic scattering. 
 
     
     
       27. A system according to  claim 26  further comprising a neutron source mounted within the tool housing and adapted to emit neutrons into the subterranean formation so as to produce gamma-rays. 
     
     
       28. A system according to  claim 27  wherein the neutron source is a pulsed neutron source.